The general objective is to determine the relationship between mutation rate and average fitness. In particular, there will be a test of the mutation load principle of Haldane, which asserts that fitness declines in proportion to the mutation rate. Since there is good evidence that most selection against mutant genes is in the heterozygous state, the study will emphasize mutant heterozygotes. Our main interest is in mutations with individually small deleterious effects rather than more drastic mutants, since the former are much more frequent in occurrence. Special stocks of Drosophila melanogaster have been developed which maintain a chromosome permanently in heterozygous condition, either in males or females. We shall measure the effect of having many mutants on a chromosome by the ability of flies carrying such a chromosome to compete with otherwise identical flies with a normal chromosome. In particular, we wish to see whether there is a linear or a more complicated relationship between the number of heterozygous minor mutants and the reduction of fitness. A direct empirical test of the relationship between mutation rate and fitness will be made by putting two species of approximately equal fitness into competition when one is exposed each generation to EMS treatment. Since many experiments in population genetics, including some of ours, involve the use of population cages, we will also measure some of the demographic parameters of caged Drosophila populations, particularly the mean age of reproduction.